As a technique to improve an output of an engine, known is a method which compresses intake air with a turbocharger and supplies an engine with the compressed intake air (supercharging). This method is widely used for automobile engines and the like. A turbocharger normally includes a rotary shaft, a turbine wheel disposed on an end side of the rotary shaft, and a compressor wheel disposed on the opposite end side of the rotary shaft. The rotary shaft rotates at a high speed in response to exhaust energy of exhaust gas being applied to the turbine wheel, and thereby the compressor wheel disposed on the opposite end side of the rotary shaft compresses intake air.
An engine equipped with a turbocharger can achieve a high output from a small exhaust amount. Thus, in recent years, a turbocharger is mounted to an automobile engine in particular to reduce the size of the engine, in order to improve fuel consumption. In this context, needs toward turbochargers for an engine of a small exhaust amount have been also increasing, and turbocharges have been reduced in size even further.
A turbine housing of a turbocharger is normally produced by casting. A small turbine housing has a higher surface roughness inside the housing than a large turbine housing. Further, a tongue portion of a small turbine housing is thicker due to the minimum possible thickness in casting. As a result, a small turbine housing has a greater flow resistance of exhaust gas. Moreover, a small turbine housing has greater casting errors than a large turbine housing, which increases variation in performance and flow-rate characteristics between products.